1. Field of the Invention
The invention relates to an arrangement for the monitoring of a resistance between adjacent rail-ends of a section of railway track, to a rail end-post for use in such an arrangement, and to a method for monitoring such a resistance.
2. Description of the Related Art
Railway tracks are conventionally divided into sections of track which are separated by an insulating member. The insulating member may be located between one rail of a section and the corresponding rail of the next section, or between both sets of rails of the two sections. Insulation is provided in order to enable the presence of a train on a particular track section to be detected. FIG. 1 shows a typical track signaling arrangement comprising three adjacent sections of track, namely sections n-1, n and n+1. The sections are insulated from each other on one rail 10 by so-called insulated block joints 12 and 13. The other rail 11 is, in this example, not provided with such block joints. Connected across the rails at one end of section n (similar arrangements apply to the other sections too, but are not shown) is a DC source 14 and a resistor 15. Across the rails at the other end of section n is a relay 16 whose contacts 17 are connected to suitable signaling circuits. When the track section n is clear, current I flows from the DC source 14 through the resistor 15, the rails 10 and 11 and through the relay 16, thereby operating the relay. Under these circumstances the signaling circuits give a "track clear" indication to the railway signaling system or to railway personnel. When, however, a train is situated on the section n, the axles and wheels of the train serve to provide a low-impedance shunting path for the current I so that the relay 16 de-energizes, thereby changing the signal from "track clear" to "track occupied". In some systems track signaling is effected by AC currents rather than DC.
The insulated block joints 12, 13 are normally constructed as in FIG. 2. In FIG. 2 it can be seen that two main items of insulation are provided: a so-called "end-post" 20 between the ends of the opposing rails 10n and 10n-1 and "skin" insulators 21 between the two adjacent rails and the fishplates 22 (only one is shown) which connect the rails. In addition, and not shown, insulating bushes are provided separating the fishplate bolts 23 from both the fishplates 22 and the rails 10n-1, 10n. (In an improved form of construction known as a "glued joint", the assembly described above is encapsulated in epoxy resin for extra strength.)
A common cause of track-signaling failure is a short-circuit failure of an insulated block joint which can cause the signaling circuit to show "occupied" instead of "clear". While this failsafe condition ensures the safety of the public and railway personnel traveling on the rolling stock, it does create unnecessary disruption to rail traffic so that throughput is needlessly reduced.
There are two potential short-circuit paths in an insulated block joint: firstly, a path via a single short-circuit from one rail-end to the next, adjacent, rail-end and, secondly, two simultaneous short-circuit paths from each of the two adjacent rails to the same fishplate. While it is known to monitor for short-circuits between the rails and the fishplate, there is a need to be able to predict a potential short-circuit (or low-impedance) between the rail-ends.